Atmospheric Processes (AtP) Group

The Atmospheric processes group in NCESS is actively engaged in the basic research on atmospheric clouds, thunderstorms, lightning and atmospheric electricity, and regional climate over Western Ghats to improve the forecasting of atmospheric natural hazards.The Asian summer monsoon circulation is an integral part of the large scale circulation (low level jet and tropical easterly jet) where the advection of moisture to the land area leads to the orographic lifting across the Western Ghats. The high amount of precipitation in the windward side of the Western Ghats is known for orographic lifting of saturated air. The diurnal cycle in the precipitation is largely influenced by these factors. The microphysical process inside cloud (vapour condensation and depositions, auto-conversion, evaporation, collection, accretion, riming and melting) involves the interactions with aerosol particle and dynamics. The physical processes are essentially controlled by the rate at which water vapour enters and leaves the volume of air and the phase changes within the volume. The formation of different types of clouds, rain and cloud droplet spectra, and the resultant precipitation are evolved through microphysical as well as thermodynamical processes. Based on the temperature, clouds are classified as warm (temperature above zero degree), cold clouds (temperature below zero degree) or mixed phase clouds (co-existence of super cooled and ice crystals). Orographically induced precipitation in the Western Ghats is dominated by warm clouds during the monsoon season. Charging and discharging process are initiated in the mixed phase and cumulonimbus clouds in the pre-post monsoon seasons. These factors are being investigated in details with the support of in situ and remote sensing measurements.

Lightning is one of the major natural hazards over the Indian subcontinent and the fatality rate is 0.25 million populations per year. The National Crime Records Bureau report in 2015 (NCRB, 2015) reports that 12.5 % natural accidental causalities in all India is contributed by lightning strike. The causalities in Kerala state by lightning strike is 97.5 % compared to other natural hazards like flood, land slide, heat waves and earthquake (NCRB, 2015). There are isolated ground based measurements and research on the lightning activity in India

Separation of the positive and negative charges within a cloud becomes very high which leads to exceed the insulating power of air resulting immense release of electrical energy. However, the inclusive mechanisms of cloud charging still remain elusive in tropical region. Presently, the most known mechanism is charge separation due to collision between ice crystals and graupels (relative bigger ice particles). Graupels generally gain negative charge upon collision whereas, ice crystals gain positive charge. Graupels being heavier move towards the bottom of the cloud, while lighter ice crystals remain on the top of the cloud due to combined effect of differential fall speed and updraft. This eventually leads to a net charge separation within the cloud.

The negative charge at the cloud bottom induces a step-like series of negative charges (know as stepped leader), works its way incrementally downward towards the Earth. When the lowermost step reaches within 50 meters of a positively charged object (streamer: building, tree or a person) a climbing surge of positive electricity takes place. When the streamer and the stepped leader meet, a highly conducting electrical channel between the cloud and the ground is formed. The negative charges in the cloud explosively flow to the ground. This is called the return stroke or lightning. The heat causes surrounding air to rapidly expand creating the pealing thunder (the sound that we hear) for a short while after the lightning flash. Often, several subsequent strikes will follow the same channel, with each stroke draining the negative charge from higher up in the cloud.

The observation sites:

NCESS established cloud physics observatories in the coastal, mid and high altitude locations in the Western Ghats. The observatory located at NCESS campus (8.29°N, 76.59°E) is a coastal station at an altitude of 20 m above MSL. Braemore (8.75°N, 77.08°E) is another observatory set-up at an altitude of 400 m above MSL in the Nedumangad area of Thiruvananthapuram District which is located about 40 km northeast of NCESS campus. The cloud physics observatory at Rajamallay (10.15° N, 77.02° E), near Munnar is a high altitude observatory located at an altitude of 1820 m above MSL. The observatory at Agumbe (13.58°N, 75.09°E) in Karnataka is situated on a plateau of 600 m elevation in the Western Ghats mountain range. The three observatories at NCESS, Braemore and Munnar are currently operational and the Agumbe observatory will be equipped soon. The data collected from in-situ cloud physics observatories will be used for the basic research on heavy rainfall, thunderstorm & lightning, monsoon, satellite - aircraft measurement validations and will be also utilized for modifying the parameterization schemes in weather prediction models.